The present invention relates generally to data communications, and more particularly, to a system and method for communicating special marker symbols in a signal space constellation.
The field of data communications typically uses a modem to convey information from one location to another. Digital Subscriber Line (DSL) technology now enables modems to communicate large amounts of data. Modems communicate by modulating a baseband signal carrying digital data, converting the modulated digital data signal to an analog signal, and transmitting the analog signal over a conventional copper wire pair using techniques that are known in the art. These known techniques include mapping the information to be transmitted into a multidimensional signal space constellation. In some instances, a one dimensional signal space constellation can be employed, such as in the case of pulse amplitude modulation (PAM). The constellation can include both analog and digital information or only digital information.
In the above mentioned communications system it would be beneficial to allow the transmission of additional special marker symbols in a signal space constellation. These special markers can be used to convey a number of command and control functions from one communication device to another. Constellations typically encode exactly N-bits in a constellation with 2N points. A need exists for the ability to transmit additional special marker symbols in a signal space constellation.
The present invention makes the convenient and efficient transmission of additional special marker symbols possible. The present invention provides a system and method for transmitting a plurality of additional symbols in a signal space constellation. The invention incorporates a marker encoder configured to encode an additional special marker into an additional symbol. This additional symbol, representing any one of a number of additional commands, such as xe2x80x9cStart of Messagexe2x80x9d, xe2x80x9cEnd of Filexe2x80x9d, xe2x80x9cEnd of Transmissionxe2x80x9d, xe2x80x9cIncrease Data Ratexe2x80x9d, xe2x80x9cDecrease Data Ratexe2x80x9d, xe2x80x9cSave Statexe2x80x9d or xe2x80x9cClearxe2x80x9d is added to a value of 2N and then transmitted in a signal space constellation in accordance with techniques that those skilled in the art will appreciate. The present invention includes a transmitter configured to transmit the additional symbol in a signal space constellation and a receiver configured to receive the signal space constellation containing the additional special symbol. A subtractor configured to subtract the value of 2N from the received signal so as to decode the additional symbol is also included in the receiver. A marker decoder configured to decode the additional symbol into the additional special marker is also included. Any number of additional special marker symbols may be transmitted.
In a preferred embodiment of the present invention a marker encoder receives special marker symbols such as xe2x80x9cEnd of Filexe2x80x9d, xe2x80x9cStart of Messagexe2x80x9d, xe2x80x9cEnd of Transmissionxe2x80x9d, xe2x80x9cIncrease Data Ratexe2x80x9d, xe2x80x9cDecrease Data Ratexe2x80x9d, or xe2x80x9cClearxe2x80x9d and supplies them in the form of an encoded symbol xe2x80x9cbxe2x80x9d, which is added to a value of 2N and supplied as an N+1 bit word to the register alternatively with the N bit word. The N bit word is supplied from an ISA bus to an N+1 bit register which separates the N+1 bit word into N most significant bits (msb""s) and 1 least significant bit (lsb). The encoded symbol xe2x80x9cbxe2x80x9d occupies the lsb positions such that larger numbers of special marker symbols are easily accommodated.
An International Standards Architecture (ISA) bus is an industry standard which eliminates the need for signal interfaces and is well known in the art. Based upon the data rate capacity of the channel, groups of 16 or 32 bit words are converted into frames of N bit words where N is determined by the data rate capacity of the channel. In order to allow the transmission of fractional bit rates as is known in the art of data communications, the register can optionally include a modulus converter or other means, such as constellation switching or shell mapping to enable the transmission of fractional bit rates. The N msb""s are next supplied to a mapper, which maps the signal into a signal space constellation. Optionally, an N bit word is supplied first to a scrambler. The scrambler performs an operation on the N bit word that results in a scrambled N bit word. While in the preferred embodiment the special marker symbols are not scrambled, they can be.
Next, the signal is operated upon by a phase encoder which is designed to develop a rotation vector using the 1 lsb supplied by the register. In the case of PAM, only one lsb is phase encoded. This rotation vector is combined in a rotator with the mapped N bit vector representing the N msb""s to form the signal space constellation of the present invention and creates a phase rotated positive or negative signal. Optionally, the phase encoder includes a differential encoder which encodes the 1 lsb of the N+1 bit word to develop 1 differential bit. This differential bit is added to the lsb""s supplied from the register and become part of the rotation vector.
The phase rotated signal is then modulated using either carrierless amplitude/phase (CAP) modulation or any coded or uncoded modulation scheme such as trellis coding, quadrature amplitude modulation (QAM), or pulse amplitude modulation (PAM), and then transmitted over a communication channel comprising a conventional wire pair. In the case of PAM modulation, the signal space is one dimensional instead of multidimensional as in QAM, however, the concepts of the present invention are equally applicable thereto. At a receiver, the transmitted phase rotated signal is received and demodulated in accordance with techniques that are known in the art of modem communications. A phase decoder which includes a phase slicer operates on the received signal and supplies a signal to a vector rotation operator which provides a derotation vector. The derotation vector is combined with the signal space constellation in a rotator which recovers the mapped N bit vector representing the N msb""s. Optionally, the output of the phase slicer is supplied to a differential decoder which develops 1 differential bit in order to recover the 1 lsb of the N+1 bit word. The 1 differential bit is subtracted from the output of the phase slicer and input to the lsb of a register.
The signal space constellation is then sliced, as is known in the art, in order to recover the mapped N msb""s in a register. As in the transmitter, the register can include a device such as a modulus converter, or other means such as constellation switching or shell mapping in order to allow the transmission of fractional bit rates. Lastly, the N msb""s are combined with the lsb and, if scrambled, the N bit word is descrambled in order to output an unscrambled N bit word.
The register supplies the N+1 bit word to a subtractor where a value of 2N is subtracted from the N+1 bit word. This output is then supplied to a marker decoder which will separate the special marker symbol from the received N bit word.
Various modulation techniques may benefit from the concepts and features of the present invention. For example, the present invention will function equally well using carrierless amplitude/phase (CAP) modulation, or any coded or uncoded modulation such as trellis coding, QAM or PAM.
The invention has numerous advantages, a few of which are delineated hereafter, as merely examples.
An advantage of the present invention is that it permits the transmission of special marker signals as symbols added to a signal space constellation.
Another advantage of the present invention is that it provides for seamless data rate changing by designated special symbols. As data rate increases, N increases and unique special marker symbols are retained by adding 2N.
Another advantage of the present invention is that it provides for very fast multi-point synchronization by using a simple designated special Start of Message xe2x80x9cSOMxe2x80x9d marker to signal the arrival of a new message. The special marker symbols are high power symbols developed by adding 2N to a power indexed signal constellation. They have maximum margin for distinguishing them from the idle channel signal which precedes each multi-point transmission, thus making them easy to distinguish.
Another advantage of the present invention is that it is simple in design, reliable in operation, and its design lends itself to economical mass production in modems.
Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention, as defined in the appended claims.